1. Field of the Invention
This invention relates to a vertical scrolling address generating device for generating a display address in a vertical direction to effect the vertical scrolling for respective images displayed on multiple image planes in a multiple image plane display system such as a videotex system or multiplexed text broadcasting system.
2. Description of the Related Art
As is well known in the art, an information providing system which is called a videotex system (it is also called a CAPTAIN system) by which an information center and user's terminals each having a television receiver are connected to one another via telephone lines to permit the users to take out necessary information from the information center and display the same has been widely used.
The user's terminals of such a videotex system are divided into five groups of ranks 1 to 5 according to the performance thereof, and among them, the terminals of ranks 2 and 3 are most widely used. The rank-2 terminal has an image display area of 248 picture elements in a lateral direction and 204 picture elements in a vertical direction, that is, an image display area corresponding to the display plane (standard picture plane) of a television receiver of present NTSC system.
In contrast, the rank-3 terminal has an image display area of 496 picture elements in a lateral direction and 408 picture elements in a vertical direction, that is, an image display area corresponding to four standard picture planes (the image plane having picture elements of four standard picture planes is hereinafter referred to as a high density image plane). Therefore, in the rank-3 terminal of the videotex system, in a case where a standard picture image 1 is received, the picture elements can be multiplied by four times and displayed on an image display area S as shown in FIG. 1A or they can be displayed in the form of four standard picture images 1 on the image display area S as they are as shown in FIG. 1B.
The above user's terminals of the videotex system are disclosed in, for example, "Technical Reference Material, Interface of Videotex Communication Network Service (Terminal Edition)" published from NTT, pp. 73-85 and 223-234 Aug. 25, 1984, or "Broadcasting Technology CAPTAIN System" by Hiroshi Taniike and Youji Koizumi, pp. 874 to 888, October, 1984.
In a case where the standard picture image is displayed on multiple image planes in the videotex system, it is considered that the standard picture image transmitted from an information center together with "multiple image plane display" specifying information attached thereto is automatically displayed on multiple image planes according to the multiple image plane display specifying information by the user's terminal or that the standard picture image transmitted from the information center without "multiple image plane display" specifying information attached thereto is displayed on multiple image planes by the operation of the user for setting the user's terminal into the multiple image plane display mode.
In general, "vertical scroll" specifying information is not included in the standard picture image transmitted from the information center, if information designating multiple image plane display is included in the image. That is, the standard picture image having the information designating multiple image plane display provides a desired image display effect when it is displayed on the multiple image planes. Thus is because the standard picture image has contents not required to be scrolled in a vertical direction.
In contrast, when the user operates the user's terminal to selectively set the same into the multiple image plane display mode, some of a plurality of standard picture images to be displayed on the multiple image planes may be transmitted from the information center together with "vertical scroll" "multiple image plane display" specifying information attached thereto. In this case, the vertical scrolling operation must be properly effected with respect to the standard picture image transmitted from the information center together with "vertical scroll" specifying information attached thereto even if it is set in the multiple image display state.
However, since the conventional rank-3 terminal is designed mainly for processing standard picture images having "multiple image plane display" specifying information attached thereto, it is functionally impossible to separately effect the vertical scroll with respect to each of the standard picture images displayed on the multiple image planes. Therefore, in a case where the user operates the user's terminal for the multiple image plane display so as to select the multiple image plane display mode, it is impossible to effect the vertical scroll with respect to a standard picture image contained in the standard picture images displayed on the multiple image planes even if it is transmitted from the information center together with "vertical scroll" specifying information attached thereto.
That is, in a case where the user operates the user's terminal for the multiple image plane display so as to select the multiple image plane display mode, the display position (quadrant) of that standard picture image which is to be scrolled in a vertical direction may be changed according to the way the standard picture image displayed on the multiple image planes is selected or the arrangement of the standard picture images and cannot be determined, and therefore, it is impossible to separately and vertically scroll only that one of the standard picture images displayed on the multiple image planes which has "vertical scroll" specifying information attached thereto if a function of separately and vertically scrolling the standard picture images displayed on the multiple image planes is not provided for the user's terminal.
Further, the above problems occur not only in the user's terminal of the videotex system but also in level-B receivers which are one type of television receivers of the text broadcasting system, for example. The text broadcasting system is disclosed in "Text Broadcasting Technical Handbook Edited By Broadcasting Technology Developing Conference" published by Kenroku Kan, pp. 13-43, 233-237 and 239-243. Aug. 1, 1986.